PODB is a PHP Object Persistent Database.

PODB is implementing persistency by reachability approach.

Object of any class derived from Persistent base class is considered as persistent capable.

To update the database, a transaction has to be started. Starting the transaction must be done explicitly; there is no implicit transaction when accessing the database. Only one transaction at a time can be opened on a connection. 

The storage has one special root object. Root object is the only persistent object accessed in the special way (using get_root method). All other persistent objects are accessed in normal way: either by traversing by references from another persistent objects, either using ReferenceList or ReferenceMap objects.

Unlike many other OODBMS, there can be only one root in the storage. If you need to have several named roots, you should create Persistent object with attribute RelationMap object and use it as root object.